saw boats

[Ian Inverarity]

Hey Aaron,

Care to discuss the design of your SAW boat? Was it scratch built, based on something else or have you modified an existing boat? Do you use rear sponsons, and if so in what configuration? Considering SAW boats often run with no fin and a faily small rudder, what have you done to compensate for prop walk? Have you concentrated on aerodynamics or light weight or both? Have you incorporated any aerodynamic downforce? Do you use a small fuel tank? Have you made the sponsons adjustable for height and angle? What sort of sponson angles did you use? How did you design the sponsons?..... I could go on! ;D

Ian.

 

[Glenn Quarles]

Hi guys, sorry I haven't answered many questions lately...I've been busy putting on a big race here and haven't had time to do much extra stuff. I will try to remember the questions asked, but if I forget something please ask again.

I like the conventional collett type of cable arrangement. It has worked well for me over the years and I believe the ball bearings in the engine carrying the thrust make for a pretty efficient system.

Yea, I like solid mounts for my engines. I think it puts a little more of the power to the prop, but I'm not sure that it is that much.

The small cable on our .12 boat was just something I tried because I couldn't get it running very well with the 3/16 I built the boat with. It still didn't run well even after I changed to the smaller cable. I finally found the problem (prop/pipe) and the speed went from the mid 50's to the mid 70's. I have not tried to go back to the 3/16 cable to see if it works better (do you think I'm CRAZY?).

Actually I don't have any problem at all with launching or running very slow with my SAW boats. For both my .21 and .12 boats I launch them by just a gentle "slide" onto the water at idle. Within a couple of feet they get themselves on plane (in fact my .21 boat will sometimes start to hit the pipe at idle if I let it run for a long distance straight) The secret to having a boat run good at both slow speeds (i.e. launch) and fast speeds is LIGHT WEIGHT. A light boat will have no problem running at slow speeds on narrow sponsons because it doesn't have to support much weight.

Weight is REALLY the important thing in getting a SAW boat to work...keeping a light boat on the water after it gets fast is another subject all together!

Yes props are VERY important! I can't tell you what prop to use for your boat, but some general guidelines are, high pitch, really thin, and really sharp. The thin and sharp has a big effect on handling as well as speed. If your boat prop walks a lot (or little) you may be able to get rid of some of it by sharping the prop more. A sharp blade "cuts" into the water while a blunt blade walks across the water as it tries to "pound" it's way in.

Glenn

 

[Ian Inverarity]

Glenn,

I have been reading a bit about your race on rcboat.com, sounds like it was very good!

Light weight, got it! ;D

An interesting point for me is narrow sponsons. Some of these boats have narrow sponsons, is that to reduce lift or reduce drag? Because some boats, like SG's and Hammer's designs, make the left front sponson riding surface narrower than the right sponson to increase the drag on the left hand side and conpensate for prop walk. Thus it seems that narrower sponsons make more drag. ???

Do you use rear sponsons on your SAW boats?

Ian.

 

[Glenn Quarles]

Ian, the width of the riding surface has to match the weight of the boat. Yes if you put narrow sponsons on a heavy boat they will tend to "sink" causing more drag. The relationship between, weight, riding area, and angle of attack are all very important. If you could see Ross' .12 boat running you would see almost zero distrubance on the water where the front sponsons ride and almost zero spray coming off of the front sponsons...this is what tells me everything is close to being right. If there is much spray or obvious "tracks" in the water where the sponsons passed then something needs to be changed.

No I don't use back sponsons on any of my SAW boats. The props provide PLENTY of lift by themselves.

Glenn

 

[Hammerhead]

Hey Glen! I look for the tracks and the water spraying of my sponsons, too. My new .12 seems to just skitter across the water in the straights. The left sponson sprays water in the corners, but I think that is okay. I also can see what you mean by the thin props not walking. When I use a 937 (which in my book is almost like tin foil) I get no torque pull at all. I have my strut at 90 degrees, and my rudder is perfectly straight when I let my hand off the wheel, and it will run straight as an arrow with a V937. Do you get a little cavitation on the launch with a 937? I did get just a tiny bit of pull with the S215, though. Nothin' I couldn't erase with a little fore finger trim adjustment. A few beeps was all it took. After I got it out of the water, The rudder still looked staight in the neutral position.

Hammer

 

[Ian Inverarity]

Hammer,

I agree on how well the thin sharp props work, the propblem is they are not very durable! Only takes a little bit of rubbish in the water to make a mess of the prop.... : I go for thin and risk the damage... ;D

Glenn,

Riding area combined with width and velocity determines the lift of the riding surface. Years ago I read an artical by an English racer on his design on a 7.5cc hydro, published in 1989. He used it to establish the British record at about 60mph, but he had the boat over 70mph in testing. He said the optimum planing angle for sponsons was 3.5 to 5 degrees and implied it was possible to calculate how large a riding surface was needed. Is this possible, or does everyone just estimate and adjust their sponsons?

When I watched my old gas Cobra on calm water I could see that on the straights the left front sponson was just along for the ride, prop torque was enought that it would only touch the water occasionally in the straights!

Something I asked about before was rudder sizing and prop walk compensation. Andy Brown wrote somewhere about one of the simplest and most effective tools against prop walk was a good sized rudder! I know from experiments myself that as I made a rudder smaller I had more problems with prop walk, even tho the rudder was adequate for control for SAW application. Do you have small rudders on your boat? How do you deal with prop walk (Other than sharp props ;D ) ?

When I started in boat racing in 1997 there was no official 100mph records. People had speeds that were close. Then it seemed that suddenly quite a few people have done 100mph or more, and now Andy Brown has 125mph and I read that Mark Grim did a 130mph one way pass! What has been the break thru in getting these speeds? Clearly it is not power as I believe the first 100mph record was set with a 45!

Ian.

 

[Glenn Quarles]

Hammerhead, it sounds like your boat is running just right. Yea the V937's are nice and thin! I haven't noticed any cavitation...they seem to hook up as soon as they hit the water. If you are just having a little bit it doesn't sound like a problem. They are good props.

Ian, I haven't done much playing with rudder widths...for the last several years all I use is Speedmaster's mini rudder. I cut the length shorter, but I have never tried one any narrower. For our new .12 SAW I have a much smaller rudder (weight) so I may find it is a problem. I will let you know.

On my last .21 boat that held the record I actually had an adjustment to angle the strut left and right a small amount and I was able to fine tune the prop walk until it ran straight.

Glenn

 

[Ian Inverarity]

Glenn,

I can shim the strut on my current boat, but I made the strut too weak, so I can actually bend it to compensate for prop walk!

Can't think of anything else to ask right now! :-

Ian.

 

[TimD]

I can!!!

Glenn,

what do you think about motor angle in the tub and stuffing tube shape - particularly with the smaller displacement motor's like the .12's and .21's???? Is it that critical to power loss in the driveline?

 

[Ian Inverarity]

Tim,

I think this would be a lot more important on the small boats, particularly with a flex hex setup. I have S bend setups in a couple of boats (monos) and they dont feel as free as simple single bends.

I would be interested to see what Glenn has found tho! ;D

Ian.

 

[Glenn Quarles]

Guys, I have always made a nice gentle bend in the stuffing box to keep the drag down on the cable (Very important in the small motors!!). BUT a good friend of mine, Rod Geraghty, recently did some testing and found that under load (spinning) having an slight "S" bend in the stuffing box actually cut down on drag. It doesn't feel like it when you turn it over with your fingers, but at RPM and under the thrust from the prop it seems to keep the cable from "whipping" and keeps the number of contact points between the cable and stuffing box to a minimum.

In any case, it is important to keep any tight bends where the cable might bind out of the drive line.

Glenn

 

[Ozzy]

Glenn

Have you got any pics of sponson shapes that you might want to share with us?

I find one of the most difficult things to get write when designing a new boat is the position of the trailing edge of the sponsons how do you approached this measurement and get it write or do you get it with trial and error.

Paul

 

[Glenn Quarles]

Paul, I don't have any pictures or measurements right now, but a good place to start with the "wheel base" length of a hydro (the length from the rear edge of the riding surface of the front sponsons to the front of the prop) is about 2/3 of the total length.

For example, if the total length of the boat (front sponson tip to prop) is 33 inches then the distance from the rear edge of the front sponsons to the prop should be around 22 inches (for this example the front sponsons would be 11 inches long).

This is just a general rule and may be adjusted for what you want a specific boat to do. Just like a car, a longer "wheel base" will tend to make the boat rider smoother over rough water, handle high power engines easier, etc.

When I get a chance I will send some measurments from one of my boats.

Glenn

 

[Ian Inverarity]

Glenn,

Looking at various hydro designs I have seen some with sponsons whose riding surface edges are parallel when viewed from underneath, eg SG eagles. Others have the outer edge tapering in towards the front, eg Hawk or Roadrunner.

I'm guessing that the designs with parallel sponson edges have less drag because the parallel edges dont push water aside when it is running, the sponsons simply climb on top of the water. The designs with the tapering edges would also be pushing some water aside when they run, hence more drag. But these designs would be inherantly more stable, the outer edges acting a bit like toe-in on a car. I speculate that this is the reason that SG's have a much wider stance and afterplane than many other designs, to use size to generate more stability. It was interesting to see that pic that Gene posted showing the new rear sponsons on the SGX, they taper in as well. I am guessing this is to make the boat more stable.

What do you think of my postulating?

Are your SAW boats using parallel sponson edges? What about your heat racing designs?

Ian.

Nitrocrazed racing: Trying to keep this thread going......

 

[Chris Attebery]

Ian,

I think that tapering the front edge of the rear shoes on the SG reduces lift in rough water and helps the boat stay level. Since the front third of the sponson is tapered I doubt it has any effect in smooth water. Just my theory.

Chris

 

[Glenn Quarles]

Ian, I have never thought about it like that. With only the last inch or two of the sponsons actually touching the water (and all of the boats I have seen have at least the last few inches parallel) I don't think this makes much of a difference. I design all of my sponsons (SAW and racing) with a tapered front for two reasons...1) I like the way it looks 2) I want to get rid of a little area at the front to help control lift.

With the amount of sponson area that touches the water (both smooth and rough water) I don't consider this a very important part of a design.

Glenn

 

[Ian Inverarity]

Glenn,

I just checked the front sponsons on my Hummingbird and they become closer to parallel towards the rear, but still have some taper all the way back to the trailing edge. From memory the Hawk design is like this too. However the Cobras and Stingers I have seen do straighten out towards the trailing edge.

I understand your point about reducing area near the front tips to reduce lift.

But in chop quite a bit of the sponsons comes into use, not just the last couple of inches. And I would think the tapered sponsons would have a bit more drag due to having to push water aside when going through choppy water. That said my Hummingbird (with the tapered sponsons) is easily the fastest boat I have driven!

Anyway my main point was one of stability. My 90 Hummingbird, which was custom made to be larger than the 60/80 design, is smaller than my friends 67 SG in afterplane and stance!

I dont know how important the taper is. Some boats have it quite pronounced while others dont. Looking at pictures of modern Cajun Bullets they have very parallel riding surfaces. I guess each person's design theory is different...... 8)

Chris,

You may be absolutely correct! Sounds perfectly plausible to me.

Ian.

 

[Don Templeton]

Hi Gleen,

I'd really like to have your best guess on this one!

(.12 rigger, flat bottom, T.E. of fronts set at 5/8"depth)

At what depth would you design the center line of the shaft (or bottom

of a 5/16 diam. strut tube, " if you prefer") to be set at?

Also, if you were running rear sponsons, what depth would you set them at?

Thanks again,

Don

P.S. I understand that Gary Pruse (Spelling-?) also likes a slight S-Bend

in his stuffing tubes for the same reason you explained above.

 

[Glenn Quarles]

Don, I always start my strut adjustment so the boat sits level on a table on the front sponsons (no turn fin) and bottom of the strut. While doing this I also adjust my strut so it is completely flat on the the table. This is not the perfect setup for every boat, but it should be very close most of the time. In fact, Ross' .12 boat is set up this way and is not even adjustable!

As you know I don't run rear sponsons on my SAW boats, but I think a good place to start is about an 1/8 inch or maybe a little bit more above the level of the table. The angle of attack on the rears (fronts too!) dependend on a lot of things so it would be hard for me to recommend anything now.

Hope this helps! If not, send me another question.

Glenn

 

[EatMyShortsRacing]

Glenn,

A while ago in Australia there were a lot of 'stick' boats made for SAW. Very long and very narrow. Is there any advantage in long narrow boats for SAW?

EMS Racing SW3 upside down